bug#54905: Allow in tabulated lists to resize columns with the mouse

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: laszlomail@protonmail.com,  54905@debbugs.gnu.org
> Date: Thu, 14 Apr 2022 18:29:32 +0200
> 
> Yes, this doesn't seem to work:
> 
> (keymap-local-set "<header-line> <drag-mouse-1>" #'foo)
> (defun foo (e)
>   (interactive "e")
>   (message "Hello %S %S"
>          (posn-x-y (event-start e)) (posn-x-y (event-end e))))
> 
> Dragging something in the header line does not call this function.

That's because we intervene, AFAICT.  Type "C-h c", then drag the
mouse on the header-line, and see what Emacs tells you about that.